The field of this invention relates to a radio frequency transmitter, an integrated circuit device, a wireless communication unit and a method therefore. The invention is applicable to, but not limited to, a method of generating a radio frequency signal for transmission over a radio frequency (RF) interface.
Advances in the deep sub-micron CMOS (Complimentary Metal-Oxide Semiconductor) process have lead to digital circuits becoming smaller and more power efficient. However, it is known that analogue circuits do not scale particularly well with the deep sub-micron CMOS process. It is therefore desirable for devices, such as radio frequency (RF) transmitters, to remove as many analogue components or circuits as possible, for example with the assistance of digital signal processing algorithms, in order to be able to benefit from more use of deep sub-micron CMOS processes.
Furthermore, a large number of conventional RF transmitters use linear power amplifiers. Accordingly, the power efficiency of such conventional RF transmitters is usually very low, due to the low efficiency of the linear PAs used therein. Switch-mode PAs have very high efficiency in comparison, which make such switch-mode PAs an attractive alternative to conventional linear PAs within RF transmitters. Thus, an RF transmitter that is able to utilize switch-mode PAs through the assistance of digital processing algorithms in order to reduce a PA's size and improve a PA's power efficiency is highly desirable. However, switch-mode PAs normally exhibit a highly non-linear input-output relationship. Furthermore, in order to meet stringent co-existence requirements of various wireless standards, noise shaping techniques are often required.
Digital polar transmitters are a type of known transmitter design that utilizes switch-mode PAs, whilst also taking advantage of CMOS process technology. Accordingly, such digital polar transmitters are able to achieve high power efficiency, whilst requiring only a small silicon area. However, a problem with these known transmitter designs is that, due to the inherent bandwidth expansion characteristics of the AM (amplitude modulation) and PM (phase modulation) signals in a polar architecture, they are only suitable for narrowband modulated signals.
Hybrid polar transmitter designs take advantage of two dimensional (in-phase/quadrature) modulation to enable wideband phase modulation to achieved. However, a problem with such hybrid polar transmitters is that they suffer from both amplitude and phase quantization noise, thus requiring significant noise shaping.
In-phase/Quadrature (IQ) RF digital-to-analogue converter (DAC) based transmitters are also known. I/Q RF DACs combine the functionalities of a DAC and a mixer, with the output of the I/Q RF DAC being combined in the analogue (RF) domain. However, such transmitter designs require a linear PA, and direct I/Q RF digital-to-analogue conversion is less power efficient than a digital polar transmitter design.
Another known (predominantly narrowband) RF transmitter design utilizes adaptive pre-distortion using a delta-sigma modulator for automatic inversion of power amplifier non-linearity. Such a design is relatively simple and allows for a use of low-precision DACs. However, this design still comprises a generally conventional architecture, and so PA efficiency is low.
It is anticipated that digitally-assisted/digitally-intensive RF transmitters will become increasingly desirable. However, digital algorithms are limited by the availability of circuit speed; therefore finding simple and effective digital algorithms is crucial from an implementation perspective.
Referring first to FIG. 1, there is illustrated a simplified block diagram of an example of a digital-to-RF converter 100 for performing modulation of a radio frequency (RF) signal. The digital-to-RF converter 100 is arranged to receive an RF signal, which in the illustrated example comprises a constant envelope RF signal 110, perform modulation of the received RF signal in a digital-to-RF transmitter 140 in accordance with a received digital codeword signal 120, and to output a modulated RF signal 130 accordingly. The digital-to-RF transmitter 140 does not have an analog base-band signal that is typically present in conventional transmitters. In contrast, a digital codeword signal (digital control word) 120 is mixed with the RF signal directly. The output waveform is an RF modulated signal 130.
FIG. 2 illustrates the known architecture for generating and applying a suitable digital codeword signal (digital control word) 120 to the digital-to-RF transmitter 140. A digital pre-distortion (DPD) codeword is created in module 205 and input to a first digital power amplifier circuit (DPA 1) 210. The DPD codeword created in module 205 is also input to a second digital power amplifier circuit (DPA 2) 220 via a delay 215 to create a quadrature version of the DPD codeword 205. The outputs from the first digital power amplifier circuit 210 and the second digital power amplifier circuit 220 are applied to a summing module 230 and the summed RF power amplified signal is output 235.
One problem associated with every digital RF transmitter is that digital codewords that control any analog block inside a digital RF transmitter only change at a certain frequency. Thus, the RF output spectrum of every digital RF transmitter presents a periodic spectrum repetition due to digital sampling, commonly referred to as digital-to-analog conversion (DAC) images, as illustrated in the signal waveforms 300, 305, 310, 315 of FIG. 3. This results in the so-called DAC images 320 that are separated by the sample frequency in RF output spectrum. Such DAC images may violate power spectral density (PSD) requirements, or in-device co-existence requirements. In conventional RF transmitters, an analog low-pass filter is typically employed immediately after the DAC in order to attenuate the DAC images. However, in digital RF transmitters, there is no such analog baseband filtering capabilities. Therefore it is important to mitigate the effect of DAC images (for example by attenuation through an RF filter) in order to prevent or minimize any PSD violation at a reasonable cost.
Thus, a need exists for an improved RF transmitter, and method of operation therefore.